1. Field of the Invention
This invention relates generally to one-chip calculators, such as those of the hand-held type, and means for effectively driving a keyboard and an associated display directly from the chip.
2. Brief Description of Prior Art
One conventional circuit for driving an LED display and an associated keyboard array requires that a plurality of external buffer transistors and current limiting resistors be utilized to accommodate the relatively high peak currents which are otherwise generated to illuminate the light-emitting diodes of the display. For example, the peak currents which may pass through both the strobe driver transistor and a segment select transistor may be between 30-40 ma. and 4-5 ma., respectively. Because of the relatively incompatible low voltage requirements of the light-emitting diodes of the display in comparison with the available voltage at the output of the strobe drivers of the conventional circuitry, it is difficult to employ the same strobe drivers for interfacing with said driving both the display and associated keyboard. As a consequence, this conventional circuit has the disadvantage of requiring additional transistors which results in relatively high space consumption and corresponding increased costs per circuit. Relatively high voltage supplies are also required to maintain a bright display, because the display otherwise tends to become dimmed as an associated supply voltage connected to one of the buffer transistors (such as a battery voltage) becomes subsequently dimmed with the continued passage of time.
In another conventional circuit, the more light-emitting diodes being energized, the greater is the loading applied to the keyboard input. Hence, reference signal levels which are indicative of the opened and closed switch positions of the keyboard keys become relatively indistinguishable from one another. As a result of the increased loading, the presence of additional keyboard driver transistors are frequently required in order to obtain an accurate indication of a designated key means switch position. Consequently, the additional number of pad connections resulting from the added keyboard driver transistors further undesirably increases the size and cost of the circuit.